The aliphatic esters and polyesters of substituted sterically hindered hydroxy-hydrocinnamic acid are well-known as effective antioxidants for a wide variety of organic materials protecting them from oxidative and thermal degradation. Many of these esters have gained wide commercial acceptance as phenolic antioxidants.
An important known class of transesterification catalysts which may be used to prepare the above compounds include tin catalysts, particularly organotin catalysts. For example, U.S. Pat. No. 4,594,444 teaches a process for the preparation of sterically hindered hydroxyphenylcarboxylic acid esters by the transesterification of the corresponding methyl or ethyl ester with a higher aliphatic alcohol using an oxide or an organometallic compound of a metal of the fourth main group or subgroup of the periodic table as catalyst in an amount between 0.05 and 1.0 mol percent based on the methyl or ethyl ester. Higher dialkyltin oxides, particularly dibutyltin oxides, are taught as the preferred catalysts for this process.
However, as recognized in the art of antioxidants, if the amount of tin residue in the product is too high, the ultimate product stability may be compromised. Care is therefore taken to remove such residues. Unfortunately, one or more of the following disadvantages are commonly associated with known methods for removing tin residue from substituted hydroxyphenylcarboxylic acid esters: product degradation and/or color formation; additional processing steps (including, for example, crystallization or adsorption techniques) which inevitably result in yield loss and increased waste generation; and increased expense due to the need for rather sophisticated equipment which may be required for the separation of product from residual tin catalyst.
Japanese Hei 01 316389 (Takee et al.) provides for the removal of organotin compounds from general ester exchange reactions by making the organotin compounds insoluble in organic media with the use of aqueous carboxylic acid solutions. In particular, the Japanese reference requires the use of an organic solvent to make the carboxylic acid derivative of the tin compound insoluble in the ester mixture, which technique increases equipment requirements as well as waste generation. The Japanese reference further requires use of an aqueous solution of a carboxylic acid. Unfortunately, the presence of a large amount of water leads to the formation of the undesirable toxic by-product, 3-(3,5-di-tert-butyl4-hydroxy)hydrocinnamic acid (HCA), the presence of which becomes problematic for the instant hydrohydroxycinnamate esters when employed in food contact applications. The reference further teaches the added step of using auxiliary filter aids, such as cellulose or activated charcoal, which are needed in order to effectively remove the reacted tine compound.
Clearly, need continues to exist in the industry for a simplified and improved process of removing residual tin from the substituted higher aliphatic esters of hydrohydroxycinnamic acids made by the tin catalyzed transesterification reaction between the corresponding lower alkyl ester and higher alkanol. It is the object of the present invention to satisfy this need.